Morphine-induced analgesia and explosive motor behavior in an amphibian.

Morphine sulfate is a weak analgesic in the frog Rana pipiens pipiens, causing a slight increase in nociceptive threshold at a dose of 10 mg/kg and a pronounced increase at 100 mg/kg. Morphine-induced analgesia persists for at least 165 min and is significantly attenuated by naloxone. The analgesic doses of morphine are well below the lethal dose and are without noticeable effect on the behavior of the frogs or their responses to non-painful stimuli. Higher doses of morphine (320 and 640 mg/kg) induced a state of hyper-responsiveness to sensory stimuli similar to the explosive motor behavior induced in rats by microinjection of morphine into the periaqueductal gray.